1. Field of the Invention
The present invention relates to variable magnification optical systems which are constituted by plural lens groups and capable of magnification by varying the spacing of the respective lens groups in the optical axis direction, an image taking lens device including the variable magnification optical system and digital apparatus incorporating the image taking lens device.
2. Description of the Related Art
In recent years, widespread use of portable phones and portable information terminals (PDA: Personal Digital Assistant) have been remarkable, and furthermore such devices incorporating a compact digital still camera unit or digital video unit have become common. Since there are severe constraints on the size and cost of these devices, it is common to employ an image taking lens device including a single focus optical system consisting of one to three plastic lenses and an image pickup device having a small size and a small number of pixels in comparison with digital still cameras and so on which are independent products.
However, such a single focus optical system has magnification ratios equivalent to those of vision and enables photographing of only objects near the photographer. In view of this point, at the present time where high-pixel and high-function image pickup device have been rapidly developed, there is a need for a compact variable magnification optical system which is applicable to a high-pixel image pickup device, capable of photographing an object far from the photographer and mountable to a portable phone, etc.
There have been various types of suggestions about such variable magnification optical system, as the following patent literatures JP-A No. 2002-169089, US-A No. 2002-149857, US-A No. 2003-58548 and US-A No. 2004-80655 for example.
The US-A No. 2003-58548 describes a variable magnification optical system including a first lens group having a negative optical power and second, third and fourth lens groups having positive optical powers, which are placed in this order from the object side, namely having a so-called four components which are negative, positive, positive, positive components, wherein the refracting powers and the materials of the respective lens groups are determined for the sake of providing excellent optical characteristics to the zoom lenses over the entire magnification range and wide angles of view while simplifying the lens system.
The US-A No. 2002-149857 describes a variable magnification optical system with three components, or negative, positive and positive components, wherein the refracting power of the second lens group is determined for the sake of compactness of the variable magnification optical system. The JP-A No. 2002-169089 discloses a technique which, in a variable magnification optical system with two components or negative and positive components, provides at least one aspherical surface in a negative lens in the latter group, for the sake of compactness of the variable magnification optical system.
The US-A No. 2004-80655 suggests a technique which, in a variable magnification optical system with two components or negative and positive components, places a light-quantity adjusting filter at a different position from the aperture stop having a fixed shape for the sake of alleviating the influence of diffractions and reducing the total optical length.
The patent literatures JP-A No. 2002-169089, US-A No. 2002-149857, US-A No. 2003-58548 and US-A No. 2004-80655 have points to be improved respectively. Namely, the variable magnification optical system in the USP-A No. 2003-58548 includes many lenses and thus is inferior in the compactness, which makes it difficult to mount it on a portable phone or portable information terminal which are required to be miniaturized. The variable magnification optical system in the US-A No. 2002-149857 is made compact to some degree. However, in order to mount it on a portable phone or portable information terminal, the compactness thereof must be further enhanced. If an attempt is made to enhance the compactness of the variable magnification optical system of the US-A No. 2002-149857, this will increase the difficulty of fabrication of lenses due to the high decentration error sensitivity of the first lens group.
The variable magnification optical system of the JP-A No. 2002-169089 causes large residual aberrations and particularly causes insufficient corrections of astigmatisms. With the variable magnification optical system of the US-A No. 2004-80655, the error sensitivity of the second group becomes extremely high, thus making it difficult to adjust the spacing of lenses and also fabricate aspherical lenses.